In recent years, active flow control has been used to increase the aerodynamic efficiency of machines having air flow over a surface, in particular vehicles such as airplanes. Adverse fluid flows generated over aerodynamic surfaces can buffet and fatigue downstream structures exposed to the flows, and the flows can affect efficiency by increasing drag or resistance over the surface. In one version of active flow control, jets of air are blown into the path of the adverse fluid flows to mix with the flows and cause the air to flow more smoothly over the aerodynamic surfaces and reduce the drag and resistance over the surfaces. In many cases, such active flow control can be implemented in existing vehicle designs without needing significant changes thereby directly reducing the operating cost of the vehicle or other machine.
One device for creating jets of air in active flow control is a synthetic jet that forms a jet flow by moving air back and forth through a small opening of the device. Synthetic jets typically have a housing in the shape of a hollow box or cylinder with a resonant chamber therein and an orifice or nozzle opening through one of the side or end walls. At least one wall of the synthetic jet is formed from a flexible membrane that can deflect inwardly and outwardly to alternately decrease and increase the volume in the resonant chamber and expel and draw in air through the opening. Deflection of the membrane may be caused by a piezoelectric actuator that responds to an applied electric field. The piezoelectric actuator may include one or more piezoceramic disks attached to the membrane.
For each piezoceramic disk, electrodes are attached to the opposing planar surfaces of the disk for application of the electric field across the thickness of the plate. Due to the converse piezoelectric effect, the applied electric field causes stresses and mechanical deformation through the thickness of the plate, and corresponding stresses and mechanical deformation occur in the plane of the plate due to the Poisson effect. In-plane deformation of the plate creates bending moments on the membrane and deflection of the membrane relative to the resonant chamber. Alternating the polarity of the electric fields across the plates causes the plates to alternately compress and elongate. Alternating the electric field at a high frequency causes rapid vibration of the membrane and creation of high velocity flow through the opening of the synthetic jet.
Two actuator topologies are available commercially. The first topology, or unimorph, attaches a single piezoceramic wafer to a metallic substrate that extends to form a passive outer ring. The metallic substrate provides a passive structure to react with the piezoelectric induced strain which bends the unimorph. The unimorph topology has reduced performance because a significant amount of piezoelectric induced strain energy is used to bend the stiff metallic substrate of the unimorph, and consequently is not available to perform work to generate puffs of air from the synthetic jet. The second topology, or bimorph, involves bonding two piezoceramic wafers together with a thin layer of epoxy and Kapton®. The outer surface of the bonded wafers is encapsulated in Kapton®. A passive outer ring is formed by adding a filler between the outer edges of the piezoceramic wafers and the outer diameter of the bimorph actuator and providing a constant thickness across the device.